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Diploma in Environmental Science - Measuring Pollution

Very small quantities of some chemicals can have a large impact on organisms. Because of this, substances that are present in trace amounts, such as nutrients and contaminants, are usually measured and recorded using very small units. Two of the most common measures are parts per million and micrograms per milliliter.

Micrograms per milliliter, or ug/mL, measures mass per volume. It is generally used to measure the concentration of a substance dissolved or suspended in a liquid.

One microgram is one millionth of a gram (1 µg = 0.0000001 g), and one milliliter is one thousandth of a liter.

Parts per million, abbreviated as ppm, is a unitless measure of proportion. It is obtained by dividing the amount of a substance in a sample by the amount of the entire sample, and then multiplying by 1,000,000 (one million).

In other words, if some quantity of gas, liquid, or solid is divided into one million parts, the number of those parts made up of any specific substance is the ppm of that substance.

For example, if 1 mL of gasoline is mixed with 999,999 mL of water, the water contains 1 ppm of gas.

Since a microgram is one millionth of a gram, and a milliliter of water equals one gram of water, ug/mL is equivalent to parts per million.

Ppm is also equivalent to many other proportional measurements, including milligrams per liter (mg/L), milligrams per kilogram (mg/Kg), and pounds per acre (lb/acre). But parts per million is often more useful in describing and comparing trace amounts of chemicals because it eliminates specific units and is applicable to liquids, solids, and gases.

Because many toxins begin to have negative environmental effects at very low levels, their abundance in ppm or ug/mL are used to set the limits of pollutants that are legally permitted in stack smoke, discharge water, soil contamination, and so on.

For example, coal fired power plants may be limited to a discharge of 0.5 ppm of sulfur dioxide in the stack smoke. If a plant\'s emissions exceed that amount, it may be in violation of local or federal air quality standards and could be subject to a fine.